The Zählwerk Enigma was developed in 1928 as an improved
version of the Enigma D.
The machine features a cog-wheel driven wheel stepping mechanism
and has a character counter (German: Zählwerk) to the left of the wheels.
The official model number was A28 and it was given the internal
designator Ch.15 by the manufacturer.
It was also known as Zählwerksmaschine[4].
A later variant of this machine, the Enigma G (G31)
became known as the Abwehr Enigma.

The image on the right shows the Zählwerk Enigma with its top lid open,
giving a clear view at the cipher wheels. The wheels, that have
the 26 letters of the alphabet engraved around their circumference,
are driven by a cog-wheel mechanism that features irregular stepping.

A crank can be inserted into a small hole to the right of the cipher
wheels, allowing the stepping mechanism to be controlled manually;
forward and backwards. This enabled the operator to correct mistakes
but also allowed manual stepping to be part of the cryptographic key.

A four-digit counter to the left of the cipher wheels was used to
count the number of characters in a message. The number of characters
was often sent in the preamble of the message and was mainly used for
verification. As the counter can not be reset, the user has to write down
the number at the beginning of the message and subtract it from the
final value at the end.

The diagram below shows the various features of the Enigma A28.
Its shiny cipher wheels and ditto wheel setting windows are clearly
visible at the upper part of the top lid. The wheels are driven by
a cog-wheel based mechanism that can be locked/unlocked by means of
a lever at the top left. In addition, a crank (stored inside the
case lid) can be used to move the wheel stepping mechanism back and
forth when making corrections. It should be inserted into a hole to the
bottom left of the power selector. A counter at the left keeps track
of the number of characters.

Inside the top lid are three metal shields. The one at the top reads
MADE IN GERMANY in English. This was done as the machine was
offered internationally before WWII. The shield at the bottom provides
details about the manufacturer Chiffriermaschinen AG and its address
in Berlin. 1

Around 1933, the name Chiffiermaschinen AG was changed to
Heimsoeth un Rinke.

History

Around 1927, Enigma manufacturer Chiffriermaschinen AG
started the development of a series of new machines, all derived
from the commercial Enigma D.
For the Reichswehr (the predecessor of the Wehrmacht),
they developed the Enigma I and for various
other customers (both civil and military) the
Enigma K family was introduced.
At the same time, development started on an improved
- mechanically more advanced - machine that was described
to customers [5] as:

Translated: Lamp-Enigma with counter and coupled cipher wheels.
It has a cog-wheel driven wheel-turnover mechanism that features
irregular stepping, making it cryptographically much stronger than
the regularly stepping Enigma D.
Several models were based on this new concept.

The Missing Link

For a long time, people have wondered why the Enigma G
(model G31 from 1931, also known as the Abwehr Enigma)
is so different from the other models like the Enigma D
and the Enigma-I,
and how it was developed.
Patents DE534947
and DE579555 (1928) were known of course,
but the machine described in those patents, is very different from
the later model G31.

All that changed when, in 2007, we discovered an original Zählwerk Enigma
that matches the patent descriptions of 1928. The machine, with serial
number A865, was probably built in 1928 and was sold to a
commercial customer in The Netherlands. It is shown in the image.

It is extremely well built, using only the highest quality components and
is clearly based on the chassis of the earlier
Enigma D (1926).
In fact, it looks just like an Enigma D at first glance. It has the
same dimensions, shows four coding wheels and has shiny metal parts.
Like in the Enigma D, the rightmost three wheels are the actual coding
wheels, whilst the leftmost one is the reflector.

Unlike the Enigma D however, in which the UKW can be set to any of 26
positions, the UKW of this machine is moved during encipherment.
Furthermore is has a counter (Zählwerk)
to the left of the wheels
and it is the first machine that features a cog-wheel driving
wheel-turnover mechanism. Although the construction is
different, the operation of the Zählwerk Enigma is identical to that of
the later Enigma G.

At the top left of the machine is a metal lever
with two settings: AUS/EIN (OFF/ON).
This is used to disengage the cog-wheels, so that
the cipher wheels can be moved freely in order to set them to
the required start position (Grundstelling or Base Key).
The crank
(used to wind the entire wheel stepping mechanism back and
forth) is stored in the bottom right corner of the top lid.

Interior

The first thing that catches the eye after opening the cover of the
Zählwerk Enigma, is the beautiful cog-wheel driven wheel-turnover
mechanism. Although the diameter of the wheels is identical to that
of the earlier Enigma models, the stepping mechanism is completely different.

The picture on the right gives a clear view of the coding wheels,
the movable UKW (on the left), the counter and the hole for the
crank. This image shows a striking resemblance with the drawings
in patent DE579555[2].

It is the first machine that features the cog-wheel stepping
mechanism, a movable UKW and multiple turnover notches.
As it is the design on which the later model G31 is based as
well, we can regard the Zählwerk Enigma as the ancestor of the
Enigma G.
It seems that by (re)discovering the Zählwerk Enigma,
we found the missing link.

More detailed pictures of the interior of this machines can be
found below.
When the wheels are removed, we get a good view of the
Eintrittswalze
(entry wheel, ETW).
Surrounding the ETW is the main driving cog-wheel. Behind the
ETW is the stepping lever that moves a saw-teeth wheel on
every key-press.
Coupled with the ETW are several smaller cog-wheels that allow
the crank to be used to wind the entire mechanism back and forth.

The smaller cog-wheels also drive a thin axle that, in turn,
drives the counter at the left. Just behind the counter is the UKW
that can not be removed from the machine. To the left of the UKW
is the usual rounded lever,
that is used to disengage the UKW,
so that it can be moved to the left, allowing removal of the
coding wheels.

Behind the UKW is the cog-wheel
coupling lever. When it is in
the EIN position (ON),
all wheels are coupled and can be moved
during encipherment. When the lever is in the AUS position (OFF),
the coupling is disengaged and the wheels can be moved freely,
allowing the message key to be set.
When the machine is closed, the
coupling lever protrudes a hole
in the metal cover.

The wheels of the Zählwerk Enigma have the same dimensions as those of the
earlier machines. The outer diameter is approximately 100 mm (4 inches) and
the hole for the spindle is also the same. The positions of the 26 contact
pads and the spring-loaded contacts on the other side of the wheel,
are identical as well. The wheel-stepping notches are completely different
however.

The image on the right shows the left sides of the wheels of a Zählwerk Enigma
(left) and a standard service Enigma - Enigma I -
(right) side by side.
The rightmost wheel shows a single triangular gap at the top,
which carries on the wheel on its left by means of pawls and rachets.

In contrast, the circumference of the wheel of a Zählwerk machine (left)
has a series of notches.
These notches can be regarded as the a large cog-wheel
with 52 teeth, of which a number of teeth are missing.
The number and spacing of the notches are different on each wheel.

The cog-wheels of all wheels are coupled via a spindle at the rear.
This spindle has three small cog-wheels with a series of alternating
long and short teeth.
The coupling can be engaged and disengaged by using the EIN/AUScoupling lever at the top left of the machine.

In order to increase the cipher period of the machine, each wheel has a
different number of notches, all being relative primes of 26. Furthermore,
there is no common factor between the numbers.
Wheels I, II and III have 17, 15 and 11 notches respectively.
Apparently, the position of the notches was never changed, as all known
Zählwerk machines, including the later G31 models,
have these notches at the same position.
The codebreakers at BP called it the 11-15-17 machine.

Ordinary Enigma wheels (left) and the wheels of a Zählwerk Enigma (right)

The Zählwerk Enigma is clearly based on the design of the
Enigma D,
albeit with some additional features and improvements. Most of these
features are described in the two German patents
DE534947[1]
and DE579555[2]
.
The most striking difference with the other Enigma models is the way
in which the wheels are moved. In the earlier Enigma D (and also in
the later Enigma I that was used by the German Army),
the wheels are moved by means of pawls, rachets and notches.
As a result, the wheel-stepping mechanism of these machines can only
move forward.

In the Zählwerk Enigma, however, the wheels are moved by a cog-wheel
based gearbox. The number of notches on each
wheel has been increased drastically, and is different for each wheel.
The longest cipher period is obtained when different prime numbers
(relative to 26) are used for the number of notches on each wheel.
And this is exactly what is done in this machine.

Another difference with the Enigma D is that the UKW (reflector)
can not only be set to any of 26 positions,
it is also moved during encipherment.
The three coding wheels are mounted on a spindle,
just like in most other Enigma machines,
whilst the UKW is fitted permanently.

Each cipher wheel has a full cog-wheel with 52 teeth attached to its
right side. On the left of the wheel is another cog wheel with the same
spacing, but with a number of teeth missing.
The presence of a pair of teeth
is equivalent to a gap on an ordinary wheel.
When the wheels are engaged, they are coupled by means of 4 small cogwheels
with teeth of alternating length.

Ordinary Enigma wheels (left) and the wheels of a Zählwerk Enigma (right)

As a result, the entire mechanism can be stepped forward and backward,
without losing the relation between the position of the wheels. A crank
can be inserted into a hole in the body of the machine, allowing the
mechanism to be wound back to the desired position.
This was used to correct mistakes,
but could also act as part of the crypographic key procedure.

The wheels of the standard Zählwerk machine have the same diameter
as the wheels of other Enigma machines, such as the
Enigma D.
With the later G31 model however,
smaller wheels were supplied as illustrated in the drawing below.
In order to accomodate the spring-loaded contacts,
they are arranged in a zig-zag pattern and the contact pads at the other
side have an oval shape.

The wheels of the initial Zählwerk machine (left) and the Enigma model G31 (right)

Most machines were supplied with just 3 wheels that could be mounted on
the spindle in 6 different orders (3 x 2 x 1).
These wheels (I, II and III) had 17, 15 and 11 notches respectively.
The positions of these notches are
identical for all recovered machines, regardless of their wiring and
regardless of the customer.
It is known that some machines were supplied with more that three wheels.
The Hungarian Army, for example had Enigma G31
machines with five wheels.

It is very likely that the Zählwerk Enigma was originally intended for
commercial use, as the wiring of the UKW and the coding wheels of some
recovered machines is identical to the wiring of the commercial
Enigma D.
In some cases, the wheels were rewired by the customer, but in many
cases the wiring of the UKW was left unchanged.
The Abwehr ordered unwired wheels [6].

Wiring

When our Zählwerk Enigma (A865) was found, the UKW and all cipher wheels had
the same wiring as the (commercial) Enigma D. This confirms that the
Zählwerk Enigma was in fact a commercial Enigma machine.
The machine (and also the later G31)
was also sold to the military
(e.g. to the German secret service, the Abwehr)
and to some foreign customers.
Some of the latter changed the wiring of the cipher wheels, but in
most cases the wiring of the UKW was left unaltered.

Wheel

ABCDEFGHIJKLMNOPQRSTUVWXYZ

Notch

Turnover

#

ETW

QWERTZUIOASDFGHJKPYXCVBNML

I

LPGSZMHAEOQKVXRFYBUTNICJDW

ACDEHIJKMNOQSTWXY

SUVWZABCEFGIKLOPQ

17

II

SLVGBTFXJQOHEWIRZYAMKPCNDU

ABDGHIKLNOPSUVY

STVYZACDFGHKMNQ

15

III

CJGDPSHKTURAWZXFMYNQOBVLIE

CEFIMNPSUVZ

UWXAEFHKMNR

11

UKW

IMETCGFRAYSQBZXWLHKDVUPOJN

Related machines

Early Zählwerk Enigma (1927)
This machine was built before the patents of its design were
actually filed and should probably be regared as a pre-production
series. The counter was placed to the right of the cipher wheels
and the wheels each had a single turnover notch, which means that it
featured regular stepping[3].
Only a few of these machines were ever built.

Zählwerk Enigma, model A28 (1928)
This machine is identical to the drawings in German patents
DE534947
and DE579555.
The cipher wheels are coupled with a gearbox-like mechanism and
each wheel has a different number of turnover notches, all
relative primes of 26. The latter greatly increases the
cipher period. On this model, the counter is located to the left
of the cipher wheels. The A28 is described on this page
and is an extremely rare machine.

Zählwerk Enigma, model G31 (1931)
In 1931 a smaller version of the Zählwerk Enigma was introduced.
These machines not only had a smaller body, but were also equipped
with smaller cipher wheels.
At the same time the battery compartment
was removed and the counter was moved to the right again.
These machines all had serial numbers starting with the letter 'G',
which is why they are often called Enigma G.
These machines are also known as Abwehr Enigma.

Enigma Z, model Z30 Mk.II (1930)
Although not really a Zählwerk Enigma, the second version of
the numbers-only Enigma (Enigma Z), is clearly based on the same
cog-wheel driven wheel stepping mechanism.